Non-controlling Interest (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 132.4	$ 133.9
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 34.7	$ 35.8
OneBeacon [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	24.30%	23.90%
Total non-controlling interests	$ 250.7	$ 244.6
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 16.5	$ 16.6
MediaAlpha [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 11.0	$ 11.7
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	5.00%	5.00%
Total non-controlling interests	$ 1.0	$ 0.7
AWG Dewar
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	18.80%	18.80%
Total non-controlling interests	$ 3.4	$ 3.9
Buzzmove [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	29.10%	29.10%
Total non-controlling interests	$ 2.8	$ 2.9
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (153.0)	$ (150.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	100.00%
Total non-controlling interests	$ 0.0	$ 4.4
Star & Shield Insurance Exchange | Total mutual and reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ (153.0)	$ (146.5)